The present invention relates to an asymmetric porous polytetrafluoroethylene membrane and a process for preparing the same. More specifically, the present invention relates to an asymmetric porous polytetrafluoroethylene membrane having desirable and excellent liquid-liquid separation performance (hereinafter referred to as “separation performance”) in addition to known characteristics of porous polytetrafluoroethylene membranes, such as water permeability resistance, gas permeability, sealing ability and electrical properties, and a process for preparing the same.
Porous polytetrafluoroethylene membrane (hereinafter referred to as porous PTFE membrane), which has excellent chemical resistance and high tensile strength, is preferred for various purposes such as filters for gas or liquid, water vapor permeable and water-impermeable membrane preparation for clothes and sheets of medical use, as well as sealing or gasket for piping or production facilities in the fields of chemical, food and semiconductor industries.
Such porous PTFE membranes and processes for preparing the same are disclosed in many patent publications such as U.S. Pat. No. 3,953,566, U.S. Pat. No. 4,187,390 and U.S. Pat. No. 5,098,625. According to those publications, a process for preparing a porous PTFE membrane generally comprises, extruding a PTFE paste comprising a mixture of PTFE fine powder and an extrusion aid such as naphtha; rolling the extrudates; removing the extrusion aid from the rolled article; then expanding the article uniaxially or biaxially; and heat-setting the expanded membrane at a temperature range from 35° C. to the melting point of PTFE in order to maintain the shape of the expanded porous PTFE membrane.
Porous PTFE membranes obtained by the above process are preferably used for sealing materials, filters, water vapor permeable and water impermeable materials for clothes and the like, but have not been used for particular application such as liquid-liquid separation where other polymeric membranes are utilized.